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decompyle3

A native Python cross-version decompiler and fragment decompiler.
A reworking of uncompyle6_.

I gave a talk on this at BlackHat Asia 2024 <https://youtu.be/H-7ZNrpsV50?si=nOaixgYHr7RbILVS>_.

Introduction

decompyle3 translates Python bytecode back into equivalent Python
source code. It accepts bytecodes from Python version 3.7 on.

For decompilation of older Python bytecode, see uncompyle6_.

Why this?

Uncompyle6 is awesome, but it has a fundamental problem in the way
it handles control flow. In the early days of Python, when there was
little optimization and code was generated in a very template-oriented way, figuring out control flow structures could be done by simply looking at code patterns.

Over the years, more code optimization, specifically around handling jumps, has made it harder to support detecting control flow strictly
from code patterns. This was noticed as far back as Python 2.4 (2004), but since this is a difficult problem, so far it hasn't been tackled
in a satisfactory way.

The initial attempt to fix to this problem was to add markers in the
instruction stream, initially this was a COME_FROM instruction, and
then use that in pattern detection.

Over the years, I've extended that to be more specific, so
COME_FROM_LOOP and COME_FROM_WITH were added. And I added checks
at grammar-reduce time to make try to make sure jumps match with
supposed COME_FROM targets.

However, all of this is complicated, not robust, has greatly slowed down deparsing and is not really tenable.

In this project, we began rewriting and refactoring the grammar.

However, even this isn't enough. Control flow needs
to be addressed by using dominators and reverse-dominators, which the python-control-flow_ project can give.

This I am finally slowly doing in yet another non-public project. It
is a lot of work. Funding in the form of sponsorship while greatly
appreciated isn't commensurate with the amount of effort, and
currently I have a full-time job. So it may take time before it is
available publicly, if at all.

Requirements

The code here can be run on Python versions 3.7 or 3.8. The bytecode
files it can read have been tested on Python bytecodes from versions
3.7 and 3.8.

Installation

You can install from PyPI using the name decompyle3::

pip install decompyle3

To install from source code, this project uses setup.py, so it follows the standard Python routine::

$ pip install -e .  # set up to run from source tree

or::

$ python setup.py install # may need sudo

A GNU Makefile is also provided, so :code:make install (possibly as root or
sudo) will do the steps above.

Running Tests

make check

A GNU makefile has been added to smooth over setting up and running the right
command, and running tests from fastest to slowest.

If you have remake_ installed, you can see the list of all tasks
including tests via :code:remake --tasks

Usage

Run

$ decompyle3 compiled-python-file-pyc-or-pyo

For usage help:

$ decompyle3 -h

Verification

If you want Python syntax verification of the correctness of the
decompilation process, add the :code:--syntax-verify option. However since
Python syntax changes. You should use this option if the bytecode is
the right bytecode for the Python interpreter that will be checking
the syntax.

You can also cross-compare the results with another Python decompiler
like unpyc37_ . Since they work differently, bugs here often aren't in
that, and vice versa.

There is an interesting class of these programs that is readily
available to give stronger verification: those programs that, when run, test themselves. Our test suite includes these.

And Python comes with another set of programs like this: its test
suite for the standard library. We have some code in :code:test/stdlib to
facilitate this kind of checking too.

Known Bugs/Restrictions

We support only released versions, not candidate versions. Note however
that the magic of a released version is usually the same as the last candidate version prior to release.

We also don't handle PJOrion_ or otherwise obfuscated code. For
PJOrion try: PJOrion Deobfuscator_ to unscramble the bytecode to get
valid bytecode before trying this tool; pydecipher_ might help with that.

This program can't decompile Microsoft Windows EXE files created by
Py2EXE_, although we can probably decompile the code after you extract
the bytecode properly. Pydeinstaller <https://github.com/charles-dyfis-net/pydeinstaller>_ may help with unpacking Pyinstaller bundlers.

Handling pathologically long lists of expressions or statements is slow. We don't handle Cython_ or MicroPython, which don't use bytecode.

There are numerous bugs in decompilation. And that's true for every
other CPython decompilers I have encountered, even the ones that
claimed to be "perfect" on some particular version like 2.4.

As Python progresses, decompilation also gets harder because the
compilation is more sophisticated and the language itself is more
sophisticated. I suspect that attempts there will be fewer ad-hoc
attempts like unpyc37_ (which is based on a 3.3 decompiler) simply
because it is harder to do so. The good news, at least from my
standpoint, is that I think I understand what's needed to address the
problems in a more robust way. But right now, until such time as
the project is better funded, I do not intend to make any serious effort
to support Python versions 3.8 or 3.9, including bugs that might come
in. I imagine at some point I may be interested in it.

You can easily find bugs by running the tests against the standard
test suite that Python uses to check itself. At any given time, there are
dozens of known problems that are pretty well isolated and that could
be solved if one were to put in the time to do so. The problem is that
there aren't that many people who have been working on bug fixing.

You may run across a bug, that you want to report. Please do so. But
be aware that it might not get my attention for a while. If you
sponsor or support the project in some way, I'll prioritize your
issues above the queue of other things I might be doing instead. In
rare situations, I can do a hand decompilation of bytecode for a fee.
However, this is expensive, usually beyond what most people are willing
to spend.

https://github.com/andrew-tavera/unpyc37/ : indirect fork of https://code.google.com/archive/p/unpyc3/ The above projects use a different decompiling technique than what is used here. Instructions are walked. Some instructions use the stack to generate strings, while others don't. Because control flow isn't dealt with directly, it too suffers the same problems as the various uncompyle and decompyle programs.
https://github.com/rocky/python-xdis : Cross Python version disassembler
https://github.com/rocky/python-xasm : Cross Python version assembler
https://github.com/rocky/python-decompile3/wiki : Wiki Documents that describe the code and aspects of it in more detail

.. _PJOrion: http://www.koreanrandom.com/forum/topic/15280-pjorion-%D1%80%D0%B5%D0%B4%D0%B0%D0%BA%D1%82%D0%B8%D1%80%D0%BE%D0%B2%D0%B0%D0%BD%D0%B8%D0%B5-%D0%BA%D0%BE%D0%BC%D0%BF%D0%B8%D0%BB%D1%8F%D1%86%D0%B8%D1%8F-%D0%B4%D0%B5%D0%BA%D0%BE%D0%BC%D0%BF%D0%B8%D0%BB%D1%8F%D1%86%D0%B8%D1%8F-%D0%BE%D0%B1%D1%84
.. _Deobfuscator: https://github.com/extremecoders-re/PjOrion-Deobfuscator
.. _Py2EXE: https://en.wikipedia.org/wiki/Py2exe
.. |Supported Python Versions| image:: https://img.shields.io/pypi/pyversions/decompyle3.svg
.. |Latest Version| image:: https://badge.fury.io/py/decompyle3.svg
:target: https://badge.fury.io/py/decompyle3
.. |PyPI Installs| image:: https://pepy.tech/badge/decompyle3/month
.. _pydecipher: https://github.com/mitre/pydecipher

rocky/python-decompile3

decompyle3

Introduction

Why this?

Requirements

Installation

Running Tests

Usage

Verification

Known Bugs/Restrictions

See Also

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